In the conventional production of printed circuit boards, an etching resist pattern or a plating resist pattern is formed by screen printing, and then etching or plating of a metallic foil is carried out to form a desired conductive circuit pattern. For the purpose of prevention of oxidation of the conductive circuit pattern or maintenance of the insulating properties, a coating film of desired shape is formed on the conductive circuit pattern using a thermosetting resin composition or a photo-curing resin composition by the same screen printing as in the formation of the conductive circuit.
Recently, fining of the conductive circuit pattern, improvement of the alignment accuracy, and besides, miniaturization of mounting parts and pitch-fining of lead of IC package have been promoted, and therefore there has been required improvement of image accuracy and alignment accuracy also in the formation of insulating coating films such as solder resist coating films. On this account, as a process capable of forming a coating film with high accuracy, a process comprising using a photo-curing resin composition to form a solder resist coating film or an insulating coating film (photo process) has been widely adopted in recent years.
In the photo process, it is general that a photosensitive resin composition is applied onto a substrate to give a coating of desired size, then the coating is dried to form a photosensitive coating film, on the photosensitive coating film is placed a negative photo mask, and exposure to active rays such as ultraviolet rays and development are carried out. The photosensitive coating film is cured with active rays transmitted by the transparent portion of the photo mask, and the unexposed portion is removed by a developing solution. The photosensitive coating film formed by the above process is further cured with heat or active rays and becomes a solder resist coating film or an insulating coating film. In this process, an image can be formed with an accuracy extremely close to the photo mask, so that a fine image or high alignment accuracy can be readily obtained.
In the above process, however, a step of applying the photosensitive resin composition onto a substrate surface and a step of drying the coating to remove the solvent are necessary, as described above. In the actual circumstances, therefore, the process of FPC production becomes complicated, and it is worrisome to reduce proportion defective caused by pinholes or foreign matters. In the developing step, the uncured portion of the unexposed area is removed by dissolving it in an organic solvent, and as the solvent, a solvent of high toxicity such as 1,1,1-trichlroethane or a combustible solvent is used. Accordingly, a photo solder resist using an incombustible and low-toxicity solvent such as an alkali aqueous solution has been desired from the viewpoint of working safety.
In recent years, therefore, studies of alkali aqueous solution developing type photo solder resists have been actively made, and some compositions are disclosed. For example, in Japanese Patent Publication No. 40329/1981, a photosensitive resin composition containing as a base polymer a reaction product obtained by adding an unsaturated monocarboxylic acid to an epoxy resin and further adding a polybasic acid anhydride to the resulting product is disclosed. In Japanese Patent Laid-Open Publication No. 243869/1986, a photo solder resist composition containing as a base polymer a reaction product obtained by adding a polybasic acid anhydride to a novolak epoxy acrylate or the like and using an alkali aqueous solution as a developing solution is disclosed. However, solder resist coating films obtained from the above compositions are insufficient in the heat resistance, solvent resistance and chemical resistance, and besides it cannot be said that they are satisfactory in the insulating properties and properties relating to dielectric constant. Moreover, because of their poor flexibility and flexing properties, these films cannot be applied to flexible circuit boards such as FPC.
On the other hand, polyimide obtained by heating polyamic acid that Is a precursor of polyimide has been used for IC protective films or layer insulating films because of its excellent heat resistance, chemical resistance, dielectric properties, etc., and a photosensitive polyimide precursor capable of directly forming a pattern through exposure and development has been proposed (Japanese Patent Laid-Open Publication No. 160140/1984, Japanese Patent Laid-Open Publication No. 158237/1993). For polyimide varnish obtained by dissolving polyamic acid in a solvent, however, a high-boiling point solvent having high polarity is usually used, and the solids concentration of the varnish cannot be increased, so that the drying time is prolonged, resulting in a problem of high processing cost. In Japanese Patent Laid-Open Publication No. 170731/1986 and Japanese Patent Laid-Open Publication No. 2037/1989, examples of dry films obtained from photosensitive polyimide precursors are described. However, the production of those films is complicated and the quality control is difficult, so that the cost becomes high. Further, imidation is difficult unless a temperature of not lower than 300° C. is applied, and besides there has been pointed out a disadvantage that even if imidation is carried out, the resulting resin exhibits low extensibility. Moreover, the photosensitive acrylate that is compatible with the polyamic acid is restricted and quantitative blending is infeasible, so that it is difficult to improve resolution. Therefore, such precursors are thought to be disadvantageous particularly for forming thick films.